Various surgical procedures, specifically cardiac surgical procedures, require an extracorporeal blood bypass circuit in order to bypass the area of the surgery. In cardiac surgical procedures, a complex extracorporeal blood circuit is created to bypass the heart while the surgery is performed. A typical extracorporeal blood circuit includes a venous and a cardiotomy blood reservoir, a mass transfer device (oxygenator) for effecting the carbon dioxide/oxygen exchange, a heat exchanger regulating the temperature of the blood before it is returned to the patient, and filters to remove any debris or gas bubbles entrained in the blood. The heat exchanger is formed integrally with the oxygenator or is closely associated therewith. It is desirable that the heat exchanger have high heat transfer efficiency and a low pressure drop.
Prior art heat exchangers use a corrugated metal core as the heat transfer barrier. The metal provides a good heat transfer characteristic and the corrugations increase surface area for heat transfer while minimizing the total size of the heat exchanger. Also, heat exchangers can be counter flow, i.e., a first fluid flows along the outside of the heat exchange barrier in one direction and a second fluid flows along the inside of the heat exchange barrier in the opposite direction.
In the case of a heat exchanger for use in an extracorporeal blood circuit, blood is being either warmed or cooled as it flows along one side of the heat exchange barrier and water, functioning as the heat exchange fluid, flows along the opposite side of the heat exchange barrier. The design criteria for the blood side of the heat exchanger are very different from the design criteria for the water side of the heat exchanger. On the blood side of the heat exchanger, the pressure drop from the input to the output of the heat exchanger must be minimized in order to use the heat exchanger within the complete extracorporeal blood circuit. Also, the blood side must be designed so that: (1) blood does not stagnate which can trap air bubbles and cause clotting; (2) blood is not subjected to shear forces which cause hemolysis or otherwise damage the blood; and (3) blood is not subjected to turbulence which can produce high shear forces and cause hemolysis or other blood damage.
The water side of the heat exchanger is not subjected to the limitations of operating within a complex biological circuit, therefore, concerns of pressure drop are limited to the pump performance characteristics of the heater/cooler. Also, water or other suitable heat exchange fluid is selected because it does not decompose or break down during use. The most important issue on the heat exchange fluid side is maximizing heat exchange efficiency.
One example of a prior art heat exchanger is disclosed in U.S. Pat. No. 4,635,755. In this device, a complex water manifold is disposed within a corrugated core, creating a narrow channel along the corrugations of the inner surface of the core. Water is confined to flowing circumferentially around the core in the convolutions. Therefore, it is desirable to have a heat exchanger with a high heat transfer efficiency while having a simple, low-cost design.